A 35-year-old woman presents with sudden-onset unilateral vision loss and eye pain on eye movements. Fundoscopy reveals hyperaemic optic disc swelling with blurred margins and early peripapillary haemorrhages. Which single feature best distinguishes papillitis from papilloedema?

## Discriminating Feature: Unilateral Presentation with Pain on Eye Movement ### Key Distinction **Key Point:** Papillitis is **unilateral** and **painful** (especially with eye movements), whereas papilloedema is **bilateral** and **painless**. This is the single most reliable clinical discriminator. ### Comparative Table | Feature | Papillitis | Papilloedema | | --- | --- | --- | | **Laterality** | Unilateral | Bilateral (usually) | | **Pain on eye movement** | Present (characteristic) | Absent | | **Onset** | Acute (hours to days) | Insidious (days to weeks) | | **Visual acuity** | Reduced early | Preserved until late | | **Aetiology** | Inflammation (optic neuritis) | Raised ICP | | **Disc appearance** | Hyperaemic, blurred margins | Pale, blurred margins | | **Peripapillary haemorrhages** | May be present | Common | ### Pathophysiology **Papillitis:** 1. Inflammation of the optic nerve head (part of optic neuritis) 2. Demyelinating or infectious aetiology 3. Unilateral axonal damage → pain on eye movement (traction on inflamed nerve) 4. Early visual loss (relative afferent pupillary defect [RAPD] present) **Papilloedema:** 1. Passive swelling from raised intracranial pressure (ICP) 2. Bilateral process (both optic nerves compressed) 3. No inflammation → painless 4. Visual loss is late finding (only when ICP very high or chronic) ### Clinical Pearl **Clinical Pearl:** The presence of **pain on eye movement (ocular opsoclonus)** in a patient with optic disc swelling is pathognomonic for papillitis/optic neuritis. This is almost never seen in papilloedema. ### High-Yield Discriminators **High-Yield:** - **Unilaterality + pain** = Papillitis (optic neuritis) - **Bilaterality + painless** = Papilloedema (raised ICP) ### Why Other Options Are Incomplete - Optic disc swelling occurs in both conditions (non-discriminatory) - Blurred margins and haemorrhages occur in both (non-discriminatory) - Raised ICP is the cause of papilloedema, not a feature that helps distinguish it from papillitis at the bedside [cite:Harrison 21e Ch 379]